index.php

<?php
$url = "http://localhost:8080/geoserver/rest/styles";
$curl = curl_init($url);
$options = array(
 //HEADER
 CURLOPT_HTTPHEADER => array(
 'Content-Type: application/vnd.ogc.sld+xml',
 ),
 //BASIC認証
 CURLOPT_USERPWD=>"admin:geoserver",
 //Method
 CURLOPT_CUSTOMREQUEST => "POST",
 //Body
 CURLOPT_POSTFIELDS => file_get_contents("test.xml"),
);
//set options
curl_setopt_array($curl, $options);
// request
$result = curl_exec($curl);
$info = curl_getinfo($curl);
echo "<pre>";
echo print_r($info);
echo "</pre>";

test.xml

<?xml version="1.0" encoding="UTF-8"?>
 <StyledLayerDescriptor version="1.0.0" xmlns="http://www.opengis.net/sld" xmlns:ogc="http://www.opengis.net/ogc"
 xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
 xsi:schemaLocation="http://www.opengis.net/sld http://schemas.opengis.net/sld/1.0.0/StyledLayerDescriptor.xsd">
 <NamedLayer>
 <Name>poi_2</Name>
 <UserStyle>
 <Name>poi_2</Name>
 <Title>Points of interest</Title>
 <Abstract>Manhattan points of interest</Abstract>
 <FeatureTypeStyle>
 <Rule>
 <PointSymbolizer>
 <Graphic>
 <Mark>
 <WellKnownName>circle</WellKnownName>
 <Fill>
 <CssParameter name="fill">#FF0000</CssParameter>
 <CssParameter name="fill-opacity">1.0</CssParameter>
 </Fill>
 </Mark>
 <Size>11</Size>
 </Graphic>
 </PointSymbolizer>
 <PointSymbolizer>
 <Graphic>
 <Mark>
 <WellKnownName>circle</WellKnownName>
 <Fill>
 <CssParameter name="fill">#EDE513</CssParameter>
 <CssParameter name="fill-opacity">1.0</CssParameter>
 </Fill>
 </Mark>
 <Size>7</Size>
 </Graphic>
 </PointSymbolizer>
 </Rule>
 </FeatureTypeStyle>
 </UserStyle>
 </NamedLayer>
 </StyledLayerDescriptor>